1. Field of the Invention
The present invention relates to a semiconductor chip as is used to achieve a chip-on-chip structure by superposing one semiconductor chip on another semiconductor chip and bonding it thereto, or to achieve a flip-chip-bonding structure by bonding a semiconductor chip to a printed circuit board with the surface of the former facing the latter. The present invention relates also to a semiconductor device having a chip-on-chip structure.
2. Description of the Prior Art
One example of the structure of a semiconductor device that is devised to achieve further miniaturization and higher integration density is a so-called chip-on-chip structure in which a plurality of semiconductor chips are superposed on and bonded to one another with the surface of one semiconductor chip facing the surface of another.
In this chip-on-chip structure, as shown in FIG. 8, two semiconductor chips 91 and 92 that are so arranged as to face each other are coupled together, with a predetermined gap secured between them, and simultaneously electrically connected together by a plurality of bumps 93 provided between them. The semiconductor chips 91 and 92 thus superposed on each other are then resin-sealed, for example, in molded resin 94.
When sealed in the molded resin 94, the semiconductor chips 91 and 92 receive considerably high pressure from the molded resin 94. Moreover, if the semiconductor chips 91 and 92 have different thermal expansion coefficients, they develop strain due to stress when subjected to a large amount of heat during resin sealing. As a result, the semiconductor chips 91 and 92 are deformed in those portions thereof which are not supported by the bumps 93, and this degrades the characteristics of the devices formed on the semiconductor chips 91 and 92.
This problem is not unique to semiconductor devices having a chip-on-chip structure, but is common to semiconductor devices having a so-called flip-chip-bonding structure in which a semiconductor chip is bonded to a printed circuit board with the surface of the former facing the surface of the latter.
An object of the present invention is to provide a semiconductor chip and a semiconductor device having a chip-on-chip structure that are free from deformation resulting from mechanical pressure or stress.
To achieve the above object, according to one aspect of the present invention, a semiconductor chip designed to be bonded to the surface of a solid body with a predetermined gap secured in between has a plurality of bumps, for supporting the semiconductor chip, formed on the surface thereof that faces the surface of the solid body. Here, the bumps include functional bumps that contribute to electrical connection between the semiconductor chip and the solid body and dummy bumps that do not contribute thereto.
In this structure, on that surface of the semiconductor chip that faces the surface (substantially flat) of the solid body such as another semiconductor chip or a printed circuit board, there are formed not only functional bumps that contribute to electrical connection between the semiconductor chip and the solid body but also dummy bumps that do not contribute thereto. Thus, it is possible to distribute evenly the force that acts on the semiconductor chip when, for example, it is bonded to the solid body and then resin-sealed together therewith. This helps prevent deformation of the semiconductor chip resulting from mechanical pressure, strain due to stress, and the like and thereby permit the devices formed thereon to offer stable characteristics.
According to another aspect of the present invention, a semiconductor device having a chip-on-chip structure is provided with a first semiconductor chip and a second semiconductor chip that is superposed on and bonded to the surface of the first semiconductor chip and that has bumps formed on the surface thereof that faces the first semiconductor chip. Here, between the first semiconductor chip and the bumps formed on the second semiconductor chip are provided cushion pads that are so formed as to be thinner than the bumps formed on the second semiconductor chip.
In this structure, it is possible to absorb, by means of the cushion pads, the force that acts on the bumps when, for example, the first and second semiconductor chips are resin-sealed. Moreover, since the cushion pads are so formed as to be thinner than the bumps, it is possible to reduce the gap between the first and second semiconductor chips as compared with cases where bumps are bonded to bumps. This helps reduce the thickness of the semiconductor device. Furthermore, since the cushion pads are so formed as to be lower than the bumps, it is possible to reduce the material cost as compared with cases where bumps are formed instead.